Introduction to Systems Engineering Fundamentals
This article forms part of the Systems Fundamentals Knowledge Area. It provides various perspectives on systems, including definitions, scope, and context. The basic definitions in this article are further expanded and discussed in Types of Systems and What is Systems Thinking?.
This article asks the following question: how can the many connotations of the word system in everyday use be turned into a theoretical framework capable of supporting the application of systems thinking in such a way that it can be usefully applied to both theory and practice across all potential application domains and disciplines?
Systems Science View
The most basic ideas of a system whole can be traced back to the thinking of Aristotle and culminate in the works of the philosopher Hegel (M’Pherson 1974). Hegel’s view of holism states that an individual as part of an organic whole is fully realized only through his relationship to the whole. The generally agreed upon systems science definition of a system comes from General System Theory (GST) (Bertalanffy 1968) and considers a system as a set of inter-related elements which form a coherent whole.
Humans have used this idea to help explain the relationships between abstract ideas by describing them as a system of related concepts, rules, or ideas. Some examples of such systems are the natural number system and political systems. The SEBoK uses the notion of a “system of ideas” to help present and explain system and systems engineering (SE) knowledge.
The elements of a system may be not only conceptual organizations of ideals in symbolic form but also real objects. GST considers an Abstract systems to contain only conceptual elements and a Concrete systems to contain at least two elements which are real objects, e.g. people, information, software and physical artifacts, etc.
General System Theory considers the similarities between systems from different domains as a set of common system principles and concepts. GST starts with the notion of a system boundary defined by those relationships which relate to membership of the system. The setting of a boundary and hence the identification of a system is ultimately the choice of the observer. This underlines the fact that system is a human construct designed to help make better sense of a set of things and to share that understanding with others if needed.
For a closed systems all aspects of the system exist within this boundary. This idea is useful for abstract systems and for some theoretical system descriptions. The boundary of an open systems defines those elements and relationships which can be considered part of the system and those which describe the interactions across the boundary between system elements and elements in the environment .
While systems thinking and the systems science and systems approaches arising from it, make extensive use of abstract systems of ideas to define the concepts and principles they are based on, they do so to enable the understanding of open concrete systems within their environment.
Open Systems
The relationships between the various elements of an open system can be related to either system structure or behavior . The structure of a system describes a set of System Elements and the allowable relationships between them. System behavior refers to the effect produced when an instance of the system interacts with its environment. The set of allowable relationships between elements in a given configuration is referred to as its state
A simple classification of system elements is:
- Natural Elements, objects or concepts which exist outside of any practical human control. Examples: the real number system, the solar system, planetary atmosphere circulation systems.
- Human Elements, either abstract human types or social constructs; or concrete individuals or social groups.
- Technological Elements, man made artifacts or constructs; including physical hardware, software and information.
Laszlo summarizes the open system property of holism (or Systemic state) as a property of the eystem elements and how they are related in the system structure which leads them to create a cohesive whole (Laszlo 1972). Open Systems can exist when the relationships between the elements reach a balance which will remain stable within its environment. Laszlo describes three kinds of system response to environmental disturbance:
- Adaptive self-regulation: All systems will tend to return to their previous state in response to external stimulus.
- Adaptive self-organization: Some systems not only return to a previous state, but also reorganize to create new stable states which are more resistant to change.
- Holon: Systems displaying characteristics one and two will tend to develop increasingly complex (hierarchical) structures.
A system may be made up of a network of system elements and relationships at a single level of detail. However, many systems evolve or are designed as hierarchies of related systems. Thus, it is often true that the elements of a system can themselves be considered as open systems.
The observed behavior of a system in its environment leads to the fundamental property of emergence “Whole entities exhibit properties which are meaningful only when attributed to the whole, not to its parts…” (Checkland 1999). At some point, the nature of the relationships between elements within and across boundaries in a hierarchy of systems may lead to behavior which is difficult to understand or predict. This system complexity can only be dealt with by considering the systems as a collective whole.
Open Systems Domains
Peter Checkland (Checkland 1999) proposed a classification of systems based on purpose into five classes: natural systems, designed physical systems, designed abstract systems, human activity systems and transcendental systems. Other similar classification systems are discussed in Types of Systems.
In the SEBOK we will consider three related open system domains:
- A natural system is one whose elements are wholly natural.
- A social system includes only humans as elements.
- An engineered system is a man-made aggregation which may contain physical, informational, human, natural and social elements; normally created for the benefit of people.
These three types overlap to cover the full scope of real-world open, concrete systems.
natural systems are real world phenomena to which systems thinking is applied to help better understand what those systems do and how they do it. A truly natural system would be one that can be observed and reasoned about, but over which people cannot exercise direct control, such as the solar system.
social systems are purely human in nature, such as legislatures, conservation foundations, and the United Nations (UN) Security Council. These systems are human artifacts created to help people gain some kind of control over, or protection from, the natural world.
Note: from the definitions above Natural and Social Systems can contain only natural and human elements respectively. In reality, while it is possible to describe and reason about social systems, many of them rely on some interaction or relationship with engineered systems to fully realize their purpose and thus will form part of one or more engineered systems contexts.
engineered systems may be purely technical systems, such as bridges, electric autos, and power generation. Engineered systems which contain technical and either human or natural elements, such as water and power management, safety governance systems, dams and flood control systems, water and power safety assurance systems are often called sociotechnical systems . The behavior of such systems is determined both by the nature of the engineered elements, and by their ability to integrate with or deal with the variability of the natural and social systems around them. The ultimate success of any engineered system is thus measured by its ability to contribute to the success of relevant sociotechnical system contexts.
Many of the original ideas upon which GST is based come from the study of systems in the biological and social sciences. Many natural systems and social systems are formed through the inherent cohesion between elements. Once formed, they will tend to stay in this structure, as well as combine and evolve further into more complex stable states to exploit this cohesion in order to sustain themselves in the face of threats or environmental pressures, as well as to produce other behaviors not possible from simpler combinations of elements. Natural and social systems can be understood through an understanding of this wholeness and cohesion. They can also be guided towards the development of behaviors which not only enhance their basic survival but also fulfil other goals or benefit to them or the systems around them. The Architecture of Complexity (Simon 1962) has shown that systems which evolve via a series of stable “hierarchical intermediate forms” will be more successful and adapt more quickly to environmental change.
Some systems are created by people for specific reasons and will need to not only exist and survive, but also achieve necessary outcomes.engineered systems can be deliberately created to take advantage of system properties such as holism and stability, but must also consider system challenges such as complexity and emergence.
Whole life.....
Understanding these system concepts and associated principles forms the basis of systems thinking. An expanded discussion of these system concepts is given in Concepts of Systems Thinking.
System Definitions – a Discussion
How is a system defined in the SE literature? Systems engineers generally refer to their system of interest (soi) as “the system,” and their definitions of a “system” tend to characterize engineered systems . Two examples follow:
- “A system is an array of components designed to accomplish a particular objective according to plan” (Johnson, Kast, and Rosenzweig 1963).
- “A system is defined as a set of concepts and/or elements used to satisfy a need or requirement" (Miles 1973).
The INCOSE Handbook (2011) generalizes this idea of an engineered system as “an interacting combination of elements to accomplish a defined objective. These include hardware, software, firmware, people, information, techniques, facilities, services, and other support elements.”
However, engineered systems often find that their environment includes natural systems that don’t follow the definitions of a “system” above in that they have not been defined to satisfy a requirement or come into being to satisfy a defined objective. These include such systems as the solar system if one’s engineered system is an interplanetary spacecraft. This has led to more general definitions of a system following the systems science approach. For example, Aslaksen (2004) says a system consists of the following three related sets:
- a set of elements;
- a set of internal interactions between the elements; and
- a set of external interactions between one or more elements and the external world (i.e., interactions that can be observed from outside of the system).
This definition of a system enables people to reason about numerous classes of dynamic systems that involve engineered, social, and natural systems.
Fundamental properties of a system described in the SE literature include togetherness, structure, behavior, and emergence. These properties provide one perspective on what a system is. “We believe that the essence of a system is 'togetherness', the drawing together of various parts and the relationships they form in order to produce a new whole…” (Boardman and Sauser 2008). Hitchins (2009, 59-63) refers to this systems property as cohesion.
Natural systems and social systems often form part of the environment in which engineered systems need to exist. For a natural or social system, simply continuing to exist, and when appropriate, to adapt and grow, is sufficient. Man-made, engineered, and sociotechnical systems are created with a defined purpose (Hitchins 2009). Thus, these systems must not only be able to exist within their environment, but also do what is necessary to achieve their purpose.
System of Interest
As can be seen from the discussion above, most attempts to define the term “system” in SE either include assumptions about the system domain being considered, or are attempts to take a systems science view which risk becoming too abstract to be of practical use. A clear distinction is needed between defining "the system" to which a systems approach is applied and defining "systems" as an abstract idea which can be used to help understand complex situations.
The concept of a system helps make sense of the complexity of the real world. This is done either by creating an abstract system to help explain complex situations, such as the real number system, by creating a standardized approach to common problems, such as the Dewey Decimal System, or by agreeing on a model of a new situation to allow further exploration, such as a scientific theory or conceptual system design. People use systems to make sense of complexity in an individual way and when they work together to solve problems.
In the systems approach , a number of relevant systems may be considered to fully explore problems and solutions and a given element may be included in several system views. Thus, it is less important that “the system” can be defined than it is that combinations of systems can be used to help achieve engineering or management tasks.
The idea of a system context is used to define a system of interest (soi) , and to identify the important relationships between it, the systems it works directly with, and the systems which influence it in some way. This engineered system context relates to the systems science ideas of an open, concrete system, although such a system may include abstract system elements. There are a number of more detailed system concepts which the systems approach must also consider, such as static or dynamic, deterministic or non-deterministic, chaotic or homeostatic, complexity and adaptation, feedback and control, and more.
All applications of a systems approach (and hence of SE) are applied to an engineered systems context, and not to an individual system.
References
Works Cited
Alaksen, E. 2004. "A Critical Examination of the Foundations of Systems Engineering Tutorial". Paper presented at 14th Annual International Council on Systems Engineering (INCOSE) International Symposium, Toulouse, France, 20-24 June 2004.
Bertalanffy, L. von. 1968. General System Theory: Foundations, Development, Applications, rev. ed. New York: Braziller.
Boardman, J. and B. Sauser. 2008. Systems Thinking: Coping with 21st Century Problems Boca Raton, FL, USA: Taylor & Francis.
Checkland, P. 1999. Systems Thinking, Systems Practice. New York, NY, USA: Wiley and Sons, Inc.
Hitchins, D. 2009. “What Are the General Principles Applicable to Systems?” INCOSE Insight, 12(4): 59-63.
INCOSE. 2011. INCOSE Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.
Johnson, R.A., F.W. Kast, and J.E. Rosenzweig. 1963. The Theory and Management of Systems. New York, NY, USA: McGraw-Hill Book Company.
Miles, R.F. (ed). 1973. System Concepts. New York, NY, USA: Wiley and Sons, Inc.
Laszlo, E., ed. 1972. The Relevance of General Systems Theory: Papers Presented to Ludwig von Bertalanffy on His Seventieth Birthday, New York, NY, USA: George Brazillier.
M’Pherson, P. K. 1974. "A perspective on systems science and systems philosophy". Futures. 6(3):219-39.
Simon, H. A. 1962. "The Architecture of Complexity." Proceedings of the American Philosophical Society, 106(6) (Dec. 12, 1962): 467-482.
Primary References
Bertalanffy, L., von. 1968. General System Theory: Foundations, Development, Applications, rev. ed. New York, NY, USA: Braziller.
INCOSE. 2011. INCOSE Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.
Additional References
Hybertson, Duane. 2009. Model-oriented Systems Engineering Science: A Unifying Framework for Traditional and Complex Systems. Boca Raton, FL, USA: CRC Press.
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